The object of the present invention is to limit the dulling of wood-cutting tools resulting from wear of their cutting edge. Retarded dulling, again, implies longer tool sharpening intervals and, at the same time, prolonged tool life. The invention only relates to limiting the "normal" wear of tools used in cutting green timber, and equivalent electrically conductive organic materials. Therefore, for instance the dulling caused by types of timber with exceedingly high mineral content or by sand adhering to the wood falls outside its scope.
At an earlier occasion, more than 30 years ago, the present applicant published results obtained in laboratory experiments demonstrating that it is possible in principle with the aid of electricity to limit the dulling of tools in green timber cutting. The present procedure emerges from the applicant's new realizations, enabling the above mentioned basic insight now to be utilized in practice as well, to the purpose of extending the tool life. The fundamental idea of the invention is stated in the claim, to which reference is made here.
In the following, to begin with, are described certain results of studies from tests carried out by the applicant himself, and results published by other authors. Next is presented the applicant's new realization, which provides a clear-cut theoretical foundation for the present procedure. The utilization of the procedure is illustrated by examples from practice and by laboratory tests. Finally, a biliography is given, listing the studies to which reference is made in the disclosure following below.
The applicant's doctors's thesis work carried out at the State Institute from Technical Research, Reference 1, was fundamental for the present invention, in that a simple, yet high accurate, cutting force measuring means was developed in its connection. In conjunction with this work, the dulling of tools was already subjected to preliminary study, the increment of the main cutting force, measured under standardized conditions at regular intervals, being used as a sensitive index for the "wear", or dulling of the tool's cutting edge. The actual studies of dulling were made feasible by this method of research.
The applicant's next study, References 2a and 2b, aimed at clarification of the basic character of the dulling phenomenon. It led to the hypothesis that the wear of the tool's cutting edge is, partly at least, an electro-chemical corrosion process. This theory was proved correct by laboratory studies of green wood cutting. In these tests, one terminal of a source of DC was connected to the machine frame, and thereby to the green wood being cut, and the other terminal was over a carbon brush connected to the cutter head, electrically insulated from the machine. When the knife was connected, over the rotating cutter head, with the positive and the green wood with the negative terminal and the DC source had a voltage of 1500 V, the dulling of the knife was accelerated by a factor of about 5. Inverting the poles resulted in reduction of dulling to about 1/8; in some instances the dulling came to an end nearly altogether. In both instances the dulling of the knife without application of electricity served as control.
Reference 2c is a patent, obtained in the name of the applicant's employer at that time, the State Institute for Technical Research, applied for in Finland only, and which could not be utilized in practice. Its essential contents have been set forth in the preamble to the main claim of this application. One study also revealed that the dulling went down to about 1/5 with 500 V.
The last mentioned study, as well as certain continued tests by the present applicant under practical conditions, yielded further confirmation of the theory that has been presented concerning the fundamental nature of tool dulling. However, practical applications turned out to be rather more inconvenient than had been expected, due to the high voltage and to the difficulties experienced in insulating, with adequate safety, the cutting tools or the wood being cut. In fact, the major result of the studies, from the practical point of view, was the idea expressed by the applicant in the last run: that the service life of tools should be extended by paying particular attention in their manufacturing to the corrosion resistance of their materials.
The findings of References 2 soon became internationally well-known, in particular through the paper of Reference 2b, which was published in German. It is believed that the tungsten-carbide industry was quickest in utilizing the results of study by producing carbide brands well appropriate for use in cutting green wood also; this had not been successfully solved before the basic character of the dulling phenomenon had been elucidated.
However, numerous researchers were intrigued by direct application of electricity in wood cutting in actual practice. References 3 and 4 represent two authoritative attempts at solving the problem.
In Reference 3, the internationally renowned authority on wood cutting, McKenzie, reports on veneer peeling trials made virtually under conditions of actual practice, at the Australian Forest Products Laboratory, Melbourne. Two parallel test peelings were performed in this study. The timber to be peeled consisted of 81 cm diameter ash discs with a length of 12 cm, veneer thickness of about 1 mm, and a total of 975 m of veneer being peeled in both cases. One trial peeling was performed conventionally, without electricity. In the parallel test, a voltage of 60 V was applied between the knife and nosebar, which were electrically insulated from the machine frame. The wear suffered by the cutting edge was measured on the front surface of the short test knife, with a measuring microscope, three times in each of the two trials. The result recorded in the test indicated that when the cutting knife of the veneer lathe was connected to the negative terminal of a DC source and the nosebar to the positive terminal, the wear of the knife's cutting edge was only 40% of what it was when no electricity was applied. An embarrassing secondary effect observed in the test was spark-over of electricity between the knife and the nosebar edge through the sap flow expressed from both ends of the bolt. During spark-over, a current of about 80 mA was measured, while the current was only 10 mA when there was no spark-over. McKenzie observes, regarding the economic significance, that if the reduced wear of knifes would enable the sharpening interval to be doubled in practice as well, this would imply no less than 50% saving both in the tool costs proper and in tool maintenance costs for these lathes.
In Reference 4, the well-known veneer manufacture researcher, Lutz, reports on similar trials at the U.S. Forest Products Laboratory, Madison, in peeling full-size, 1.2 m long oak bolts. The nosebar of the veneer lathe was connected to the positive terminal and the knife to the negative terminal of a DC source, the voltage being 300 V. In addition to reduced dulling, which was not measured on the full-size knife, a further object was to prevent the marring of the veneer by blue stain discolouring it, which adheres to the veneer when the lathe momentarily stops. This is an embarrassing phenomen in the peeling of several hardwood species, one of them oak, which was used in the tests. The observation was made as a results of the tests that the electric voltage greatly reduced the blue staining of the veneer, whereas the stain transferring to the veneer from the nosebar increased. In addition, the spark-over through the sap at the ends of the bolt, mentioned in connection with the preceding study, was so strong that the sparking caused an appreciable defect at this point on the knife edge.
Reference 3 and 4 confirm the result presented in the applicant's publications of Reference 2, concerning the fundamental nature of dulling in cutting green wood, and they emphasize even further the significance of using tool materials resistant to corrosion. On the other hand, the primary objective of the studies, that is direct utilizing of electricity towards limiting tool dulling in practice, was not attained in these studies.
The next two references, though not concerning wood-cutting, gave the applicant interesting additional understanding of the behavior of wood in an electrical field.
Reference 5 concerns trials carried out at the U.S. Forest Products Laboratory, in which large-sized veneer bolts were rapidly heated to peeling temperature, utilizing the electrical resistance of the wood. AC was used in these tests at voltages ranging from 1000 to 5000 V.
Reference 6 describes drying tests with small, green test specimens (diameter approximately 75 mm, length 40 to 70 mm) making use of the electrical resistance of the wood to heat the specimens. In these tests, both conventional AC and DC and voltages from 200 to 2000 V were used.
On the basis of the measurements it can be noted that the ohmic resistance of green wood is not greatly variable. For example, in fresh-cut pine with a water content of 125% calculated on the dry weight of wood, the resistance increases approximately by a factor of 3 as the water content falls to the wood fibers' saturation point which is considered to be the limit moisture content of "green wood". This limit moisture content is about 30% for pine. With further drying of the wood, the resistance goes up steeply. The resistance increases by a factor of 1 million when the water content decreases from the wood fiber saturation to 5%. In fact, dry wood is nearly as good an insulator as glass.
The main reason why attempts to curb the dulling of cutting tools have not yielded any results so far in instances of practical cutting is the difficult task of insulating the cutting tools, or the wood being cut, and the apprehension due to the risks involved with high voltages. The actual solution of the problem was to be the present applicant's realization that after all electrical insulating neither of the cutting tool nor of the wood from the frame of the cutting machine is necessary, because even green wood has a very high specific resistance compared with metals, on the order of 10.sup.7 times as high. This implies that the DC can simply be introduced with a brush contact operating as an anode, or by any other suitable contact, in the wood being cut, close to the tool that should be protected, this tool operating as a cathode.
Once this basic insight had been reached, it was simple to present also a clear-cut theoretical background for the invention. This background may be crystallized in three main points, as follows: